Contrasting effects of soil development on hydrological properties and flow paths

Abstract

Runoff pathways strongly influence hydrologic and biogeochemical losses and landscape evolution. On an evolving landscape, soil development may alter hydrologic properties and thereby change through time the relative importance of various pathways. Here we report in situ soil water retention, unsaturated and saturated hydraulic conductivity, and flow path characteristics of a 300 year old Andisol and a 4.1 million year old Oxisol, located at the extreme ends of a soil substrate age gradient across the Hawaiian Islands. The two soils contrasted in depth and texture; the young soil was shallow and coarse textured, while the old soil was deep and highly weathered with a near‐surface plinthite horizon overlying numerous clay‐rich subsurface horizons. The young soil drained freely under modest suction, whereas subsurface clay horizons at the old site required significantly more suction to start to drain than the upper horizons. Similarly, saturated hydraulic conductivity (Ks) was high throughout the soil profile at the young site, whereas Ks was two to three orders of magnitude lower through the subsurface clay horizons than the upper ones at the old site. Irrigation experiments with deuterium tracer demonstrated that water was downward advecting at the young site, while water at the old site moved both laterally along the subsurface clay horizon contact and slowly downward through it. Rainfall frequency distributions indicated a high probability of rainfall events exceeding subsurface Ks values in old soil. In Hawaii the addition of dust influences the time evolution of soil, but the tendency for subsoil clay accumulation in older soils leading to alteration in hydrologic flow paths has been proposed in other environments. Our findings together suggest that as soils develop with time, subsurface horizon Ks values decline, impeding rates of vertical water flow but also increasing the importance of shallow subsurface lateral flow.

What makes Darwinian hydrology "Darwinian"? Asking a different kind of question about landscapes

“…At older sites the soils became increasingly weathered, producing illuviated clay horizons, which tended to reduce vertical infiltration and promote lateral redistribution. This shift in hydrologic process dominance has been hypothesized to be accompanied by an increase in the drainage density and landscape evolution (Lohse and Dietrich, 2005;Lohse, 2002). Jefferson et al (2010) noted a systematic decrease in the baseflow index and increase in drainage density as watersheds aged.…”

Chemical weathering fluxes from volcanic islands and the importance of groundwater: The Hawaiian example

“…As a consequence, flowpaths of water change (Lohse and Dietrich, 2005;Jefferson et al, 2010). On young, unweathered surfaces most rainfall percolates uninterrupted to the groundwater table, and streams do not occur on the surface.…”

A process-based diagnosis of catchment coevolution in volcanic landscapes: synthesis of Newtonian and Darwinian approaches

“…This seems counterintuitive to findings for volcanic rocks in the Hawaiian Islands, where soils on younger rocks are shallow and coarse textured, whereas soils on older 335 rock are deep and highly weathered (Lohse and Dietrich, 2005). A possible explanation for this contradiction is that the fractured bedrock and saprolite above young volcanic rock acts as a hydrologically active layer, whereas shallow clay layers in older catchments impede vertical flow, which thus result in effectively shallower soils.…”

“…hypothesized that the younger catchments exhibit groundwater-dominant response because most of the rainfall infiltrates and percolates into the permeable bedrock, which typically contains many cracks and fissures (Lohse and Dietrich, 2005). Older catchments on the other hand exhibit flashier hydrographs because less permeable clay layers at shallow depth, formed by chemical weather-…”